1. Field of the Invention
This invention relates to a method and an apparatus for assigning radio channels between base stations (BSs) and mobile stations (MSs) in a mobile communication system such as an automobile telephone system or a portable telephone system.
Recently, the number of people who want to subscribe to a mobile communication system such as an automobile telephone system or a portable telephone is increasing. However, there are only limited frequency bands available for this purpose. In view of this, it is required to meet this large demand by effectively utilizing the limited frequency bands available.
2. Description of the Related Art
In a mobile communication system such as an automobile telephone system or a portable telephone system, in order to respond to a demand which is increasing year by year, the area (zone or cell) covered by each BS is reduced by subdividing the zone and the utilization of frequencies is enhanced by repeatedly using the same frequency band since the interference is small between well spaced zones.
The most easily controllable system of those presently put into practical use is a fixed channel assignment (FCA) system. According to this system, radio channel groups are allotted to the respective zones beforehand in a fixed manner in accordance with wave propagating environments and estimated traffic distribution lest interzone interferences should occur, when the system is designed. If a channel assignment requirement is issued in response to origination or reception of a call in each zone, radio channels are assigned sequentially from the radio channel group allotted to that zone.
However, with the FCA system, it is difficult to flexibly cope with variation or temporary change of traffic due to a change in demand after the system is built or a change in the wave propagation environment caused by buildings built thereafter. Further, a division loss, which results from the fact that the channels are divided into a plurality of channel groups, leads to a limited capacity for subscribers.
Accordingly, in order to flexibly cope with the traffic variation and the wave propagation environment so as to meet an increase in the number of subscribers, a dynamic channel assignment (DCA) system is proposed in which usable radio channels are not separately allotted to the respective zones in the fixed manner, but are treated as resources shared by all of the zones.
As one type of the DCA system, for example, there is proposed a first available (FA) system in which each BS autonomously retrieves the radio channels from among the shared resources in a random sequence when an assignment requirement is issued by the BS in charge of the respective zones, and the radio channel which has first satisfied an assignment condition is assigned. The assignment condition is: the radio channel is not used in the BS and has a SNR (signal power to noise ratio) or CIR (carrier power to interference ratio) not smaller than a reference value. By setting the assignment condition in this way, more radio channels can be assigned to such a zone that must accommodate larger traffic compared to surrounding radio zones and the division loss which occurs when the FCA system is adopted can be avoided since a population of the radio channels from which the respective BSs retrieve the assignment channels includes all the system channels.
However, since the radio channels are assigned at random regardless of the distance between the BSs and MSs (or the intensity of relative electric field strength) according to this system, the channels whose conditions are needlessly good may be assigned. In this system, maximizing the accommodated traffic in the entire system is not taken into account.
More specifically, when the MS is relatively close to the BS, the received power is high at both the MS and BS and the distance of the MS from the neighboring zones is ensured to be greater than a specified value. Accordingly, it can be ensured that the MS can resist interference from neighboring zones and the influence thereof to the neighboring zones is not higher than a specified level. Thus, it is possible to repeatedly use the same channel at spatially relatively short intervals because of less mutual interference between the MSs relatively closer to the BSs. In other words, efficient assignment is possible among small size clusters corresponding to areas inward of the zones. Conversely, the same channel can be repeatedly used at geographically relatively long intervals for the MSs distant from the BSs, i.e. efficient assignment is possible among large clusters corresponding to areas outward of the zones.
In a reuse partitioning (RUP) system, a spatially efficient assignment of radio channels is realized, based on this concept, by classifying the MSs into small clusters and large clusters in accordance with the distance between the BS and the MSs, by separately allotting two different channel groups to the small clusters and the large clusters, and by assigning the channels from the respective channel groups. According to this system, an average cluster size in overall channels is smaller than when the FCA system is adopted. In other words, the number of radio channels per one zone can be increased.
However, with this system, since the radio channel groups are allotted to the small clusters and to the large clusters in a fixed manner, only the averaged traffic in the entire zone is increased, but the traffic cannot be accommodated if the traffic becomes larger than the number of channels allotted to one of the clusters, thus causing a division loss similar to the FCA system.
"Autonomous Reuse Partitioning (ARP) in Microcellular Systems" by Kanai, Technical Study Report, IEICE, RCS91-32, 1991, discloses, as one of the FA systems, a method according to which a retrieval sequence common for all the zones is determined in a fixed manner, the respective BSs autonomously retrieve channels in accordance with this common sequence, and the first found channel is assigned, consequently realizing the efficient assignment similar to the RUP system.
However, with this ARP system, since all the BSs retrieve channels in the same sequence regardless of the distance between the BS and MS, the number of channels to be searched increases as the traffic in the neighboring zones increases. Thus, this system suffers from the problem of a longer processing time and an increased frequency of required mutual communication.